On Mon, 2025-01-27 at 10:13 +0100, Benoit Lobréau wrote:
> On 1/25/25 7:07 PM, Laurenz Albe wrote:
> > Looks good to me. I have one question left: the explanation for the performance
> > penalty of a high leaf fragmentation sounds like it would only be relevant for
> > disks where sequential reads are faster. If that is correct, perhaps it would be
> > worth mentioning.
>
> Frederic noticed a performance hit even for on his laptop with a SSD.
>
> On Fri, 2025-01-24 at 15:41 +0100, Frédéric Yhuel wrote:
> > I've noticed that maximum leaf_fragmentation can have a huge impact on
> > a range index-only scan, when reading all blocs from disks, even on my
> > laptop machine with SSD, but I don't know if this is the right place
> > to document this?
>
> He reported to our team, that he did a test with two indexes on the same
> data. They had the same density but one had no fragmentation while the
> other had 100%. He got an execution time of ~90ms (0 frag) vs ~340ms
> 100% frag).
>
> I get similar result with my laptor (except my disk is significantly
> worse: ~152ms vs ~833ms).
Thanks for checking.
I'll set the patch "ready for committer".
I personally would still like to know how fragmentation slows down performance.
Yours,
Laurenz Albe
